1. Field of the Invention
The present invention relates to a demodulating system for the multivoice television system and, more specifically, to a demodulating system for the CATV system prevailing in the United States of America.
2. Description of the Prior Art
The multivoice television broadcasting prevailing in the United States of America is of the BTSC system. FIG. 2 shows the spectrum of a baseband signal of the BTSC system. The sum signal (L+R), the difference signal (L-R), the pilot signal, the SAP signal and the PRO-CH signal of an aural signal shown in FIG. 2 are transmitted after frequency modulation together with a video signal. FIG. 3 shows a multivoice broadcast receiving system. Referring to FIG. 3, in order to receive the aural signal, a channel is selected to convert the frequency of the aural signal into a corresponding intermediate frequency by a tuner 1, then the intermediate frequency signal is given to a voice demodulator 9, which demodulates the intermediate frequency signal to obtain the composite signal of the sum signal and the difference signal. The composite signal is decoded by a MPX decoder 10 to provide an L-signal and an R-signal.
The CATV broadcasting, as well as the conventional TV broadcasting, transmits signals by the multivoice BTSC broadcasting system. The CATV broadcasting employs the PAY broadcasting system (chargeable broadcasting), in which a receiver receives signals through a specific contracted channel contracted between the receiver and the station. To make impossible the unlicensed reception of a broadcast through channels other than the contracted channel, video signals are scrambled for transmission and timing pulses for restoring the scrambled video signals are superposed on the video signals through amplitude modulation. In such a CATV broadcasting system, the video signals and the amplitude-modulated timing pulses disturb the aural signals. The multivoice broadcast market, in particular, requires stereophonic PAY broadcasting, and those jamming signals need to be eliminated for the satisfactory reception of a stereophonic broadcast.
FIG. 4 shows a conventional demodulating system for the multivoice system. In FIG. 4, there are shown a tuner 1, an IF band-pass filter 2, an intercarrier demodulator 6, an intercarrier band-pass filter 7 and an FM detector 8. An intermediate frequency TV signal selected and frequency-converted by the tuner 1 is filtered by the IF band-pass filter 2 to eliminate signals transmitted from the adjacent stations which disturb the TV signal. The filtered TV signal is given to the intercarrier demodulator 6, and then the intercarrier demodulator 6 provides a signal having a frequency corresponding to the difference between the frequency of the video carrier and that of the sound carrier, namely, a beat signal. In the case of the NTSC system, the frequency of the beat signal is 4.5 MHz. The beat signal is employed as an intermediate frequency sound signal. The output signal of the intercarrier demodulator 6 is filtered by the intercarrier band-pass filter 7 to eliminate a video signal component other than the beat signal to give a beat signal having only a sound signal component to the FM detector 8, where the composite signal of the difference signal and the sum signal is demodulated. The composite signal is given to an MPX decoder, not shown, to obtain the L-signal and the R-signal. This demodulating is called as the intercarrier receiving system.
In modulating the intercarrier, namely, in generating the beat signal, by the intercarrier receiving system, the video signal is superposed on the intercarrier and is given as a disturbing signal to the 1fH, 2fH, 3fH, . . . and nfH of the composite signal to produce an annoying noise (buzzing) in the stereophonic sound. Since the 1fH of the composite signal has a disturbing signal, a disturbing signal of the same frequency as that of the pilot signal affects adversely to the MPX decoder. For example, the MPX decoder is caused to perform a stereophonic action by the disturbing signal during the reception of an ordinary broadcast or the phse of the pilot signal is deviated during the reception of a stereophonic broadcast. Furthermore, in most cases, the tuner is provided as an individual CATV converter and the composite signal is not provided during the channel selection of the CATV converter, which produces channel selection noise (pop noise).